A three-step synthesis of 4-phenyl pyridine from alphamethyl styrene, formaldehyde and ammonium chloride has been reported by Schmidle and Mansfield, J. Am. Chem. Soc., 78, 1702-1705 (1956). In that process the reactants are condensed to form 6-methyl-6-phenyltetrahydro-1,3-oxazine. This is converted with excess acid to 4-phenyl-1,2,3,6-tetrahydropyridine which in turn is dehydrogenated to 4-phenyl pyridine using nitrobenzene and palladium catalyst. The authors point out both in the foregoing paper and in U.S. Pat. No. 2,847,414 that this dehydrogenation reaction can be effected in the presence or absence of a hydrogen acceptor such as nitrobenzene. The patent indicates that the dehydrogenation should be conducted at temperatures between 125.degree. and 220.degree. C.
The condensation of 4-vinyl pyridine with diene hydrocarbons has been described by Petrov and Lyudvig, The Journal of General Chemistry of the U.S.S.R., Volume 26, January 1956 (English Translation Copyright 1956), Consultants Bureau, Inc., New York, N.Y., pages 49-51. The authors investigated the reaction of 4-vinyl pyridine with butadiene, piperylene, isoprene, diisopropenyl and cyclopentadiene. All the substances, apart from that prepared from cyclopentadiene, were subjected to dehydrogenation over palladium. The resultant arylpyridines were isolated in the form of picrates.
The use of chloroprene in Diels-Alder reactions with acrylic acid or styrene is reported by Titov and Kuznetsova, Bulletin of the Academy of Sciences USSR, Division of Chemical Sciences (English Translation, 1960, page 1687).
In order to synthesize halophenylpyridines, previous workers have resorted to reactions between a diazonium salt and a pyridine. See in this connection Butterworth, Heilborn and Hey, J. Chem. Soc., 1940, 355-358 and Netherlands Application No. 6,414,307, June 11, 1965 [Chemical Abstracts, 64 713d (1966)]. In these procedures, mixed isomers are formed and thus in order to recover the individual isomers in relatively pure form, recourse was had to such separation techniques as fractional crystallization and column chromatography.
It is known that aryl halogen atoms can be removed by hydrogen in the presence of a noble metal catalyst such as palladium. See H. O. House, Modern Synthetic Reactions, 2nd Edition, W. A. Benjamin Publishers, Copyright 1972, page 14.